Cosmological Constraints from Moments of the Thermal Sunyaev-Zel'dovich Effect

In this paper, we explain how moments of the thermal Sunyaev-Zel'dovich (tSZ) effect can constrain both cosmological parameters and the astrophysics of the intracluster medium (ICM). As the tSZ signal is strongly non-Gaussian, higher moments of tSZ maps contain useful information. We first calculate the dependence of the tSZ moments on cosmological parameters, finding that higher moments scale more steeply with sigma_8 and are sourced by more massive galaxy clusters. Taking advantage of the different dependence of the variance and skewness on cosmological and astrophysical parameters, we construct a statistic, ||/^1.4, which cancels much of the dependence on cosmology (i.e., sigma_8) yet remains sensitive to the astrophysics of intracluster gas (in particular, to the gas fraction in low-mass clusters). Constraining the ICM astrophysics using this statistic could break the well-known degeneracy between cosmology and gas physics in tSZ measurements, allowing for tight constraints on cosmological parameters. Although detailed simulations will be needed to fully characterize the accuracy of this technique, we provide a first application to data from the Atacama Cosmology Telescope and the South Pole Telescope. We estimate that a Planck-like full-sky tSZ map could achieve a <1% constraint on sigma_8 and a 1-sigma error on the sum of the neutrino masses that is comparable to the existing lower bound from oscillation measurements.Comment: 11 pages, 12 figures, to be submitted to Phys. Rev. D; v2: 14 pages, 16 figures, matches PRD accepted version (changes from v1 include additional calculations with primordial non-Gaussianity and a new appendix discussing the tSZ kurtosis

Relaminarisation of Re_τ=100 channel flow with globally stabilising linear feedback control

The problems of nonlinearity and high dimension have so far prevented a complete solution of the control of turbulent flow. Addressing the problem of nonlinearity, we propose a flow control strategy which ensures that the energy of any perturbation to the target profile decays monotonically. The controller’s estimate of the flow state is similarly guaranteed to converge to the true value. We present a one-time off-line synthesis procedure, which generalises to accommodate more restrictive actuation and sensing arrangements, with conditions for existence for the controller given in this case. The control is tested in turbulent channel flow (Re_τ = 100) using full-domain sensing and actuation on the wall-normal velocity. Concentrated at the point of maximum inflection in the mean profile, the control directly counters the supply of turbulence energy arising from the interaction of the wall-normal perturbations with the flow shear. It is found that the control is only required for the larger-scale motions, specifically those above the scale of the mean streak spacing. Minimal control effort is required once laminar flow is achieved. The response of the near-wall flow is examined in detail, with particular emphasis on the pressure and wall-normal velocity fields, in the context of Landahl’s theory of sheared turbulence

Relaminarisation of Re_{\tau} = 100 channel flow with globally stabilising linear feedback control

The problems of nonlinearity and high dimension have so far prevented a complete solution of the control of turbulent flow. Addressing the problem of nonlinearity, we propose a flow control strategy which ensures that the energy of any perturbation to the target profile decays monotonically. The controller's estimate of the flow state is similarly guaranteed to converge to the true value. We present a one-time off-line synthesis procedure, which generalises to accommodate more restrictive actuation and sensing arrangements, with conditions for existence for the controller given in this case. The control is tested in turbulent channel flow ($Re_\tau=100$) using full-domain sensing and actuation on the wall-normal velocity. Concentrated at the point of maximum inflection in the mean profile, the control directly counters the supply of turbulence energy arising from the interaction of the wall-normal perturbations with the flow shear. It is found that the control is only required for the larger-scale motions, specifically those above the scale of the mean streak spacing. Minimal control effort is required once laminar flow is achieved. The response of the near-wall flow is examined in detail, with particular emphasis on the pressure and wall-normal velocity fields, in the context of Landahl's theory of sheared turbulence

Influence of localised smooth steps on the instability of a boundary layer

We consider a smooth, spanwise-uniform forward facing step de ned by the Gauss error function of height 4-30% and four times the width of the local boundary layer thickness δ_99. The boundary layer flow over a smooth forward-facing stepped plate is studied with particular emphasis on stabilisation and destabilisation of the two-dimensional Tollmien- Schlichting (TS) waves and subsequently on three-dimensional disturbances at transition. The interaction between TS waves at a range of frequencies and a base flow over a single or two forward facing smooth steps is conducted by linear analysis. The results indicate that for a TS wave with a frequency F 2 [140; 160] (F=! =U21 106 where ! and U1 denote the perturbation angle frequency and freestream velocity magnitude, respectively), the amplitude of the TS wave is attenuated in the unstable regime of the neutral stability curve corresponding to a at plate boundary layer. Furthermore, it is observed that two smooth forward facing steps lead to a more acute reduction of the amplitude of the TS wave. When the height of a step is increased to more than 20% of the local boundary layer thickness for a xed width parameter, the TS wave is amplified and thereby a destabilisation e ect is introduced. Therefore, stabilisation or destabilisation effect of a smooth step is typically dependent on its shape parameters. To validate the results of the linear stability analysis, where a TS wave is damped by the forward facing smooth steps direct numerical simulation (DNS) is performed. The results of the DNS correlate favorably with the linear analysis and show that for the investigated frequency of the TS wave, the K-type transition process is altered whereas the onset of the H-type transition is delayed. The results of the DNS suggest that for the perturbation with the non-dimensional frequency parameter F = 150 and in the absence of other external perturbations, two forward facing smooth steps of height 5% and 12% of the boundary layer thickness delayed H-type transition scenario and completely suppresses it for the Ktype transition. By considering Gaussian white noise with both xed and random phase shift, it is demonstrated by DNS that transition is postponed in time and space by two forward facing smooth steps

Growth and optical properties of self-assembled InGaAs Quantum Posts

We demonstrate a method to grow height controlled, dislocation-free InGaAs quantum posts (QPs) on GaAs by molecular beam epitaxy (MBE) which is confirmed by structural investigations. The optical properties are compared to realistic 8-band k.p calculations of the electronic structure which fully account for strain and the structural properties of the QP. Using QPs embedded in n-i-p junctions we find wide range tunability of the interband spectrum and giant static dipole moments.Comment: Proccedings paper for MSS-13, 7 pages, 4 figure

Hydrogen bonding in cubic (H_2O)_8 and OH∙(H_2O)_7 clusters

A systematic study is presented for OH∙(H_2O)_7 clusters derived from the cubic (H_2O)_8 octamer by replacing one water with a hydroxyl radical. The system is a prototype for atmospheric water clusters containing the environmentally important OH species, and for OH adsorbed at the surface of ice. The full set of 39 symmetry-distinct cubic OH∙(H_2O)_7 clusters is enumerated, and the structures are determined using ab initio quantum chemical methods. Graph invariants are employed to obtain a unified analysis of the stability and structure of cubic (H_2O)_8 and OH∙(H_2O)_7, relating these physical properties to the various hydrogen-bond topologies present in these clusters. To accomplish this the graph invariant formalism is extended to treat a hydrogen bonding impurity within a pure water network